Ground noise reduction amplifier



Oct. '20, 1942.

J. E'HRET 2,299,385

GROUND NOISE REDUCTION AMPLIFIER Filed D90. '18, 1940 inventor Robert J Ehret I Qttomeg Patented Oct. 20, 1942 Robert J. Ehret, Collingswood, N. J., Radio Corporation of America,

Delaware assignor to a corporation of Application December 18, 1940, Serial No. 370,604

6 Claims.-

This invention relates to a ground noise reduction amplifierfor use in sound recording on photographic film or the like. Such amplifiers are of a type having an A. C. input corresponding to sound waves and a D. C. output varying in accordance with the envelope of the sound Waves.

The use of such amplifiers is not limited .to the sound recording field as they may be used for volume controls, expanders, compressors, or any equivalent purpose where a rectifying and filtering amplifier is required.

The present invention is an improvement on the type of ground noise reduction amplifier described and claimed by G. L. Dimmick in application Serial No. 358,813, filed September 28, 1940. In thesaid application'Dimmick proposed to use a rectifier in series with the loading resistor of the input transformer of the rectifier circuit so as to improve the starting characteristics of the circuit. I have discovered that the Dimmick circuit has a tendency to produce an excessive reverse voltage surge. The purpose of the present invention is to provide means for eliminating this voltage surge and thereby improve the operation of the circuit.

One object of the invention is to provide an improved ground noise reduction amplifier.

Another object of the invention is to provide an amplifier which will have desirable starting characteristics Without undesirable surges.

Another object of the invention is to provide an amplifier with A. C. input and D. C. output which will have desirable time constants.

Other and incidental objects of the invention will be apparent to those skilled in the art from a reading of thefollowing specification and an inspection of the accompanying drawing in which Figure 1 is a schematic diagram of my improved amplifier, and

Figure 2 shows a modification thereof.

In this amplifier the audio frequency input current is fed through the transformer III to the rectifier II, which is preferably of the thermionic type. The rectified current passes through a timing circuit including the capacitor I2, the resistor I3, the resistor I4, and the capacitor I5, to the amplifier I6, the output of which operates the ground noise reduction apparatus I8. The grid bias of the tube I6 and therefore the mean plate current through the apparatus I8 is controlled by the adjustable resistor I1,

In the timing circuit" the capacitor I2 is charged directly from the rectifier II and the former I0 and to prevent excessive peak volt-v ages, a resistor I9 is usually provided shunted directly across the secondary of the transcapacitor I5 is charged through the resistor I4 former I0.

In the Dimmick application above referred to,

it was proposed to provide a series of rectifiers preferably of the copper oxide type 20 in series with the resistor I9and operating in thesame direction as the rectifier II, so that the resistor I9 would have no effect until the voltage generated across the secondary of the transformer I 0 reaches the breakdown voltage of the rectifiers 20 which in the said application was indicated as being preferably of the order of 1.2 volts.- So that with the resistor I 9 having a value of 1400 ohms the effective resistance of the combination was about 10,000 ohms for voltages below 1.2 volts, while the rectifier resistance dropped to about 250 ohms for higher voltages, giving an effect of total resistance of about 1650 ohms at voltages above 1.2..

This arrangement gave an effective gain of approximately 18 db. at low amplitudes in the voltage applied to the mainrectifier II so that the shutter or other ground noise reduction de-' vice opened rapidly at these low voltages, thereby preventing clipping of peaks on rapidly rising signals.

The circuits proposed by Dimmick, although effective for its purpose, nevertheless had a substantially infinite resistance in the opposite direction so that the opposing half waves not transmitted by the rectifier II affectedthe voltages developed on the capacitor I2 through the capacitance of the rectifier II and thereby adversely affected the characteristics of the apparatus. 1

I have discovered that this undesirable effect can be avoided by adding a rectifier 2| in a direction opposite to the rectifiers 20. This rectifier 2| may, as indicated, be only a single stage copper oxide rectifier, which has a high' resistance in the direction current passes through the rectifiers 20 but which has a very low resistance in the opposite direction. As described in the said Dimmick application, battery may be substituted for the a single rectifier and a bias plurality of i rectifiers in order to determine the starting voltand the resistor l9 which give the circuit a resistance of only about 1400 ohms in this direction. This arrangement-serves to effectively suppress any undesirable voltage surges which might otherwise be produced due to the high resistance of the rectifiers 20 in the direction opposite to their normal conduction.

The modified circuit shown in Fig. 2 works in exactly the same manner as the circuit of Fig. 1 except that the battery 22 provides a bias opposing the current which would be passed by the rectifier 20 and thereby establishes the starting voltage of the rectifier 20.

I claim as myinventlon:

1. In a circuit includingv a transformer, a rectifier and a timing circuit connected to said transformer through said rectifier, the combination of a pair of reversely asymmetrically conducting paths and a resistor connected directly across the terminals of thesecondary 'of said transformer through said paths.

2. In a circuit including a transformer, a rectifier and an output circuit connected to said transformer through said rectifier, the combination of a pair of mutually exclusive conducting paths and a resistor connected in shunt to the secondary circuit of said transformer through said paths.

3. In a circuit including a transformer, a halfwave rectifier and a timing circuit connected to said transformer through said rect*fier, thecombination of a full wave rectifier and a resistor connected in series between the secondary circuit terminals of said transformer.

4. In a circuit including a transformer, a halfwave rectifier and an output circuit connected to said transformer through said rectifier, the combination of a full-wave rectifier and a series resistor connected in shunt to the secondary circuit of said transformer through said paths.

5. In a circuit including a transformer, arectifier and a timing circuit connected to said transformer through'said rectifier, the combination of a pair of mutually exclusive conducting paths and a resistor connected in shunt to the secondary circuit of said transformer through said paths, and an amplifier connected to the output of said timing circuit.

6. In a circuit including a transformer, a rectifier and a timing circuit connected to said transformer through said rectifier, the combination of. a pair of reversely asymmetrically conducting paths and a resistor connected directly across the terminals of the secondary of said transformer through said paths, an amplifier connected to the output of said timing circuit, and ground noise reduction apparatus connected to the output of said amplifien. v

ROBERT J. EHRET. 

